This invention relates to an apparatus and method for removing volatile and semi-volatile contaminants from solid materials and, more particularly, to an apparatus and method for thermal desorption of organics and volatile metals from soils to separate the contaminants from the soil.
The contamination of soils, sludges, ashes, and other solids by organics and heavy metals is a significant environmental problem. Due to the large volumes involved and expensive disposal costs for these solids, there is a need to reduce the volume of waste requiring disposal.
The contaminated soil may be treated by destruction of the contaminants, such as by incineration or by pyrolysis, or by separation of the contaminants from the soil, such as by thermal desorption. During incineration, the contaminants are heated under oxygen concentrations and residence time to a temperature effective to decompose the contaminants. During pyrolysis, the contaminants are heated in the absence of oxygen for a predetermined residence time to a temperature effective to decompose the contaminants. During thermal desorption, the contaminants are heated under oxygen concentrations and residence time to a temperature effective to avoid decomposition of the contaminants, thereby enabling the separation of the volatilized contaminants from the soil.
An example of an incinerator is the apparatus disclosed in U.S. Pat. No. 4,050,900 (Hobbs et al.), which is herein incorporated by reference for its description of an incineration system utilizing a conveyor belt. Materials are moved through a combustion chamber on a conveyor belt. As the materials advance through the chamber, the materials are incinerated, due to a combination of infrared heating means and combustion of the devolatilized organics, which releases combustion gases. Although incineration reduces the volume of waste requiring disposal, highly toxic combustion products are produced, such as dioxin, which must be effectively treated in the afterburner and scrubbing section. If fixed based incineration facilities are used, large volumes of contaminated materials must be packaged and transported to licensed incineration facilities, which have limited capacity.
The EPA Applications Analysis Report, Shirco Infrared Incineration System, June 1989, pages 39-42, discloses utilizing infrared heating elements to heat material positioned on a conveyor belt, which results in desorption of the contaminants from the material within a primary chamber, followed by incineration of the desorbed contaminants in the primary chamber. If combustion of the desorbed contaminants is not complete within the primary chamber, then the desorbed contaminants are incinerated in a secondary chamber, which produces toxic combustion products.
In another incineration process, U.S. Pat. No. 3,954,069 (Loken), sludge is indirectly heated to produce an exhaust gas of air, malodorous gases, and water, which is passed to a condenser. In the condenser, only water is condensed and removed so that the "contaminant" malodorous gas passes to an incinerator.
A process similar to incineration is disclosed in Process Technology and Flowsheets, McGraw-Hill, 1979, pages 225-226, in which waste is pyrolyzed at 900.degree. F. to provide oil vapor, gas vapor and water vapor. The vapors are quenched to form pyrolytic fuel oil and off-gas. However, during pyrolysis, the contaminants are decomposed, which may result in residual compounds, such as materials which are hazardous, toxic, or require regulation by the Environmental Protection Agency, remaining in the material.
An example of thermal treatment of soil is disclosed in U.S. Pat. No. 4,738,206 (Noland). Contaminated soil is conveyed through a chamber by a screw conveyor with internally heated flights and vapor stripped at a temperature below the boiling temperatures of the contaminants. This approach, however, will result in the production of very large volumes of gas, which then must be treated for contaminant removal.
Another example of thermal desorption of soil is disclosed in U.S. Pat. Nos. 4,977,839 (Fochtman et al.) and 4,864,942 (Fochtman et al.). Contaminated materials, which are placed in a rotary kiln, are subjected to a temperature effective to volatilize the contaminants, but below the incineration temperature, for a period of time sufficient to effect the desired degree of separation of contaminants. However, incineration is due to a combination of the presence of an oxidizing substance and the residence time of the material at a particular time, and is not due to the temperature at which the process occurs.
U.S. Pat. No. 4,782,625 (Gerken et al.) discloses volatilizing organic compounds in a rotating cylinder having plural flights for moving soil through the cylinder. The material drying means, filtering means, scrubbing means, and bed of activated carbon are individually mounted on the bed of trailers to facilitate transportation of the equipment.
Another description of processes to treat contaminated soil using a rotary kiln can be found in Contaminated Land Reclamation and Treatment, Michael A. Smith ed., Plenum Press, pages 37-90, "On-Site Processing of Contaminated Soil" by W. H. Rulkens, 1985. Incineration, treatment with catalysts, and low temperature vaporization of contaminants are disclosed. In low temperature treatment of off-gas, Rulkens discloses the use of steam as a carrier gas, cyclone filtering, condensing, gas scrubbing, separation of gas from the resulting contaminated liquid, and filtering the gas through activated carbon before discharging the gas.
However, utilization of the rotary kiln, the rotating cylinder, or the screw conveyor has several disadvantages. Due to the tumbling of the material within the drum, which may cause portions of the material to progress through the drum at different rates, the residence time of the material within the rotating drum to effect the desired degree of separation is variable.
Another disadvantage to the utilization of the rotary kiln and the screw conveyor is the limited heat transfer rates through the shell of the kiln or through the flights of the screw, which limits the throughput of these processes. Also, the soil or debris final temperature is limited by the materials of construction of the kiln and screw conveyor because the flame temperature required to obtain the desired heat transfer through the material to volatilize the contaminants may cause damage to the kiln and screw conveyor, such as material fatigue and melting of the rotary kiln and screw conveyor. Utilization of a lower temperature to volatilize the contaminants requires a longer residence time of the material within the thermal desorption unit.
Another disadvantage to the utilization of a rotating drum is the production of dust within the drum due to the tumbling of the material, which makes it difficult to treat the off-gas, produces a large amount of contaminated waste and can lead to operational problems, such as pipe blockages.
Therefore, what is needed is an improved apparatus and method that efficiently separates the contaminates from the contaminated material, that has an increased throughput, and that does not produce additional toxic materials.